1. Field of the Invention
The present invention relates to electroluminescence and is particularly concerned with the production of flexible phosphor-containing structures generating visible light by electrical field excitation, for use in lighting and display devices.
2. Prior Art
While a fairly large number of patents have been issued directed to asserted improvements in electroluminescent (EL) devices, these devices to date have not received significant market acceptance for various reasons apparently because of serious drawbacks, including but not limited to one or more of the following: high energy input required for the light intensity obtained, relatively short useful life of the light emitting element and the comparatively high manufacturing costs of certain of the proposed more complicated structures.
There prior art devices, in general, employ as the light producing element a bonded laminate comprising a layer of phosphor particles dispersed or embedded in a dielectric matrix and an electrode member bonded to each face or surface of the phosphor-containing layer in some instances by an intervening dielectric material, at least one of the electrodes being light transmitting.
Among early patented EL lamp structures of the flexible type is that described in U.S. Pat. No. 2,733,367. The light emitting element of the patent comprises, according to a preferred embodiment disclosed, a polyethylene terephthalate film (PET) containing finely divided phosphor material, such as activated zinc sulfide, dispersed in the PET film. This light-emitting element is sandwiched between aluminized PET films bonded to the light-emitting element by adhesive cement. The described laminate is energized to emit light in the visible range by connecting the aluminized surfaces to a source of alternating current at a potential in the order of 400 volts at a frequency of 10 kilocycles. In other embodiments disclosed in the patent the phosphor may be contained in other dielectric matrices, such as beeswax, solidified oils, plasticized cellulose-nitrate, etc.
The following later-issued patents are illustrative of the subsequent developing state of the art.
U.S. Pat. No. 3,052,810 advocates the use of a fluorescent material mixed with or disposed in contact with the EL phosphor layer, to absorb at least part of the light emitted by the phosphor so as to emit light at a longer wavelength than that of the absorbed light. Among the flluorescent substrates listed in the patent are rhodamine, fluorescine, and commercially available "Day-Glow" materials.
U.S. Pat. No. 3,315,111 features a flexible EL laminate asserted to be operative at 120 volts, 60 cycle AC input. The phosphor-containing layer is sandwiched between a pair of electrode layers, of which at least the front electrode has a light transmittance capacity of at least 60%. A barrier layer, composed of barium titanate dispersed in an organic polymeric matrix of high dielectric constant, such as plasticized cyanoethyl cellulose is employed as an insulation between the back electrode and the phosphor-containing layer. The matrix containing the phosphor may likewise be formed of cyanoethyl cellulose, as also is the light-transmitting lacquer electrode matrix in which electro-conductive particles, such as metal oxide pigment, is incorporated, for example indium oxide doped with a few percent of tin. The assembled laminate is encapsulated in a transparent plastic envelope.
U.S. Pat. No. 3,341,915 discloses a technique for assembling and bonding the several strata of an EL lamp element. A layer of light-transmitting electrically-conductive material such as copper iodide is deposited on a rigid solid temporary support to provide a light-transmitting electrode. A layer of phosphor-impregnated plastic dielectric in liquid state is deposited over the copper iodide, and covered, in turn, by a thin, light-reflecting back electrode of aluminum. After the phosphor-impregnated plastic layer has had sufficient time to cure and become securely bonded to the copper iodide, the formed unit is peeled from the underlying rigid support, which is made of a material to which copper iodide does not strongly adhere. The stripped self-supporting assembly is encased in a plastic protective envelope.
U.S. Pat. No. 3,580,738 asserts that the use of a thin coating of aluminum as the light-transmitting electrode of an EL device is undesirable in that the light transmission is thus reduced to unacceptable level. To overcome this drawback, the patent proposes the use as the light-transmitting electrode a thin layer of indium metal deposited on a transparent plastic. The deposited layer is heated in air and acid washed to improve electrical conductivity and light transmission. Cyanoethyl cellulose is recommended as a matrix for the phosphor-containing layer. The indium may be applied by vacuum deposition or by sputtering. A layer of high dielectric constant material (alkali earth metal titanate) in an organic binder is interposed between the phosphor layer and the back electrode.
U.S. Pat. No. 4,097,776 advocates coating of the individual granules of the EL phosphor with a liquid crystal substance. The thus coated phosphors are said to be highly resistant to moisture and certain other deteriorating influences.
U.S. Pat. No. 4,143,297 discloses a rather elaborate EL information display panel. The emphasized features of the patent are the use of as the light-emitting element a dielectric resin incorporating an unpatterned layer of EL power particles of one particle thickness, and use of a black back electrode to increase visual contrast. Fluorescent material may be included in the dielectric resin. Insulating films are interposed between the phosphor-containing resin layer and the front and back electrodes respectively.